1. Field of the Invention
The present invention relates to a new and improved apparatus and method for making molds and encompasses both conventional or green sand molding processes and the newer process known as no-bake wherein the binder is a catalyzed plastic resin material rather than the conventional natural material binders commonly used in green molding.
One of the most common and economical metal forming techniques involves the pouring of molten metal into a preformed cavity called a mold wherein the metal then solidifies. After solidification, the mold is opened and the casting is removed or knocked out of the mold for further processing and the mold is destroyed in the process. Just as the characteristics of the liquid or molten poured into the mold cavity may vary dependent on the design of the particular casting, the molding material utilized in making the mold cavity may also vary widely. At the present time, the most commonly used molding material is silica sand and the silica sand grains are bound together to provide sufficient mold strength by many types of binding materials. In green sand molding, which is by far the most widely used method in molding metal castings, the binding material comprises a mixture of clay, water and bentonite and the silica sand and binder are treated in various types of equipment until a more or less homogeneous mold forming mixture of sand and binder is achieved. A mold cavity is then formed with this mixture by depositing the mixture over a pattern which is removed to form the cavity.
The sand and binder mixture should have a number of characteristics to be successfully used for high quality castings. The mold forming material must be flowable in order to readily fill the cavities and contours of the pattern needed. The material must have sufficient green strength to retain the cavity shape while the pattern is removed until the molten metal is introduced. The material must also have sufficient hot strength to contain the high temperature molten metal within the mold cavity and provide the needed dimensional accuracy while allowing for the escape of gases which develop as the molten metal flows into the mold cavity and cools. After cooling of the casting, the material must be readily destructable so that the finished castings can be removed or "knocked out". The spent molding material, for economical reasons, must also be reworkable or reclaimable for re-use in successive mold forming operations with a minimum of refinement and treatment being required.
It should be noted that some of the characteristics required are conflicting, for example, the needed hot strength of a mold to provide for good dimensional accuracy makes the mold much tougher to destroy and the "knock out" process is difficult after the casting process has been completed and the casting has cooled. In addition, the greater amount of binder added for producing hot strength make reclamation of the molding materials more difficult and more expensive. Because of these types of conflicts, a compromise in characteristics of the molding material is usually made because all of the desired characteristics are not achievable. These same problems are present with the more modern method of "no-bake" molding wherein a catalyzed plastic resinous material is used for binder with silica sand. In "no-bake" processes high mold strength is achieved but the process is considerably more expensive because of the relatively high cost of the plastic resin binders. Also, the "knock out" process is much more difficult and the material is not generally re-usable on an economical basis. Moreover, sometimes "no-bake" process molds do not readily permit the hot gases formed during the molding process to readily escape and thus, gas pockets develop in the castings. In addition, the dispensing apparatus used in the "no-bake" molding process is subject to clogging and other problems because the mixture sets up or hardens very quickly after the catalyst is added to the sand-resin mixture.
2. Description of the Prior Art
In prior art green sand molding systems the sand and binder are mixed in batches or continuously in a constant ratio to provide the necessary compromise characteristics needed for the mold. After mixing is completed, the material is deposited into a mold flask around a pattern and subsequently, the pattern is then separated from the mold. Usually a pair of mold flasks, called a cope and a drag, are coupled together to form the completed mold cavity. The characteristics of the molding sand mixture is usually constant for a given operation and hence certain parts of the mold may have more binder than is actually needed. This, of course, results in increased difficulty in the knock-out operation and in the reclamation process to re-use the same molding sand again after the castings are knocked-out and removed. Because of the complexity of making different or varying mixes of sand and binder for different portions of a mold to provide the different strength characteristics as required in the prior art systems a uniform mixture was usually established and used throughout. Because of this, many of the foregoing problems in green sand casting occurred.
In no-bake molding systems, only relatively small batches of sand and binder are mixed at one time because of the relatively short working time available before setting of the resin takes place. Once the resin is catalyzed, the batch of material has to be placed in the molds very quickly in order that the material does not set up before the mold is completed. Usually, the whole surface of the mold pattern is covered with the molding sand and catalyzed resin mix having a relatively uniform ratio of sand and resinous material and accordingly in areas where the mold strength required is not high, excess resin is wasted with the attendant economic loss and knock-out problems. Also, since a large amount of resinous material is present throughout the mold, the organic material in the binder cannot all be oxidized during the casting operation with the result that heavy vapor and fumes are produced during the casting operation which is obnoxious to the operators and from an air pollution standpoint.
It is therefore an important object of the present invention to provide a new and improved method and apparatus for making customized nolds.
More particularly, it is an object to provide a new and improved method and apparatus of the character described which eliminates or reduces one or more of the foregoing mentioned difficulties of prior art systems.
Yet another object of the present invention is to provide a new and improved program controlled customized method and apparatus for making molds wherein a layer of molding material comprising a mixture of sand and binder is dispensed over a mold forming pattern by relative movement between the pattern and the dispenser with the ratio of sand and binder automatically and selectively controlled and varied as needed in accordance with the relative position of the dispenser and the pattern. Thus, in areas on the pattern where high stress occurs in the mold, more binder or resin is mixed with the sand and in other areas where the mold strength requirement is reduced, lesser amounts of binder or resin are utilized. This results in economic savings all the way down the line in producing finished castings.
Yet another object of the present invention is to provide a new and improved program controlled, customized method and apparatus for making molds of the character described wherein the thickness of the layer of mold forming material deposited on the surface of the pattern is automatically controlled and can be varied at different positions over the mold forming pattern in accordance with the strength required at a particular point on the pattern surface.
Still another object of the invention is to provide a new and improved method and apparatus of the character described wherein the ratio of molding sand and binder is selectively controlled and variable in response to the point of application of the molding mixture onto the surface of the pattern used for forming the mold cavity.
Still another object of the present invention is to provide a new and improved method and apparatus of the character described wherein a programmed control system of the tape or drum controlled type is provided for selectively and variably controlling relative movement between a molding material dispenser and a mold forming pattern as well as the ratio of sand and binder being deposited at any particular location.
Still another object of the present invention is to provide a new and improved method and apparatus of the character described wherein the relative position of a molding material dispensing device and a mold forming pattern surface are automatically controlled in a plural axis coordinate system and the thickness of the deposited layer of molding material is also automatically controlled.
Another object of the present invention is to provide a new and improved apparatus and method of the character described wherein either a molding material dispensing device is movable relative to a fixed mold forming pattern surface or vice versa.
Another object of the invention is to provide a new and improved mold having an inner layer formed of molding material including a wetting agent for improving the surface of the material being cast and reducing the amount of mold material tending to stick to the casting after the knock-out process and having succeeding layers around the inner layer without said wetting agent.
Yet another object of the invention is to provide a new and improved mold having an inner first layer forming a mold cavity comprising a mixture of sand and organic binder of a thickness such that substantially all of the binder is oxidized by the heat received from the molten material cast in the cavity and having successive layers around said inner layer comprising a mixture of sand and inorganic binder.
Yet another object of the invention is to provide a new and improved continuous mixer for molding material including a first portion for mixing sand and uncured resin binder and a second portion for mixing catalyst with said and uncured binder.
Yet another object of the invention is to provide a new and improved continuous mixer as in the preceeding object including a controllable metering plate for controlling the flow between said first and second portions of said mixer.
A further object of the present invention is to provide a new and improved method and apparatus of the character described wherein the successive layers of molding material which are automatically deposited on the pattern may have different organic or inorganic binders so as to facilitate knock-out and reclaiming of the mixture and to minimize smoke and fumes during the casting process.
Still another object of the present invention is to provide a new and improved method and apparatus of making molds of the character described wherein program controlled apparatus is provided for forming molds in a highly accurate and repeatable manner.